Factors influencing sediment transportation

Sediment transportation along coasts relies on the energy from waves, tides, and currents to move eroded material. This process shapes the coastal landscape, particularly in the intertidal zone, which is the area between the highest high tide and the lowest low tide.

Role of waves in transportation

Waves are a primary force in creating and altering landforms within the intertidal zone. They break on the shore, carrying and depositing sediment. Waves approaching the beach at an acute angle (a sharp angle less than 90°) transport sediment more rapidly than those arriving perpendicularly, as the angled approach enhances along-shore movement.

Influence of tides on sediment movement

Tides determine where waves break on the beach; during high tide, waves reach further up the shore. The tidal range, which is the difference between high and low tide levels, affects current strength. Larger tidal ranges, up to 16 metres in some areas, generate more powerful currents capable of moving substantial sediment. In narrow areas like estuaries, tides funnel seawater, creating especially strong currents that transport large amounts of material.

Types of currents and their effects

A current is a consistent flow of water in a specific direction, often moving parallel to the coast. Some currents are permanent, while others are temporary, lasting only hours. Rip currents form when water is channelled between underwater obstacles, accelerating its flow and carrying sediment seaward from the beach and foreshore (the area exposed at low tide) to the nearshore zone (just beyond the low tide mark).

Main processes of sediment transportation

Sediment is transported through four key processes, depending on particle size and water energy. These processes occur as water interacts with dissolved substances or solid particles.

Four transportation processes

• Solution - Soluble materials, such as limestone in slightly acidic water, dissolve and are carried within the water flow.
• Suspension - Fine particles like silt and clay are stirred up by turbulence (irregular water swirling) and remain suspended in the water column; this is the most common method for transporting eroded material.
• Saltation - Heavier particles, such as pebbles or gravel, bounce along the seabed propelled by water force, as they are too dense for suspension.
• Traction - The largest particles, like boulders, are rolled or pushed along the seabed by the water's energy.

Longshore drift and its mechanism

Longshore drift is a key process that moves sediment along the coastline in a zig-zag pattern, influenced by wind and wave directions. The prevailing wind (the most common direction) and dominant wind (the strongest) dictate wave paths, which can shift with changing wind conditions.

Steps in longshore drift

1. Swash (the forward rush of water up the beach) carries sediment, such as shingle or pebbles, parallel to the prevailing wind direction.
2. Backwash (the return flow down the beach) pulls sediment perpendicular to the shoreline due to gravity.
3. When winds create an angle between waves and the shore, repeated swash and backwash cycles shift sediment along the coast in a zig-zag manner, gradually transporting it over distance.

This process contributes to the formation of various depositional landforms by redistributing material along the shore.

Formation and features of beaches

Beaches develop through deposition, where constructive waves (waves with stronger swash than backwash) deposit sediment on the shore. This creates distinct features shaped by wave action and sediment characteristics.

How beaches form and sort sediment

Beaches accumulate when waves lose energy and drop sediment. Natural sorting occurs from the top of the beach to the shoreline: larger, more angular particles remain higher up because backwash lacks the energy to move them, while smaller, rounded particles are carried further down. This results in a gradient of sediment sizes.

Key beach features

• Berms - Ridges of sand and pebbles, around 0.8–2.5 metres high, forming at the high tide mark where waves deposit material.
• Runnels - Grooves in the sand running parallel to the shore, created as backwash drains water back to the sea.
• Cusps - Crescent-shaped dips in the beach surface, formed by waves approaching at an angle.

These features are more prominent on shingle beaches, which have steeper gradients than sandy beaches due to larger particle sizes resisting backwash.

Other depositional landforms

Deposition happens when water velocity decreases or volume reduces, lowering its capacity to carry sediment. Larger particles deposit first. Sediment sources include the seabed, rivers, and subaerial processes (land-based actions like weathering). This forms various landforms, often where the coast changes direction.

Spits and their variations

Spits are elongated ridges of sand or shingle extending from the coast, typically forming across river mouths or where the coastline bends sharply. Longshore drift deposits material, building the spit outward.

Types of spits:

• Simple spit - A straight extension roughly parallel to the coast.
• Recurved spit - Features a curved end due to occasional shifts in dominant wind and wave direction.
• Compound spit - Has multiple recurved sections if wind directions alternate repeatedly, with sheltered mudflats and saltmarshes developing behind.

When longshore drift occurs in opposing directions, it can create cuspate forelands (triangular projections from merged spits) or double spits (two spits extending towards each other across a bay, separated by river outflow).

Bars, tombolos, and barrier beaches

• Bars - Ridges connecting two headlands across a bay or river mouth, trapping seawater to form lagoons that may become less saline over time with freshwater input. Offshore bars form parallel to the coast and may remain submerged.
• Tombolos - Bars linking the mainland to an island, often a former stack (isolated rock column), through deposition or wave refraction (bending of waves around obstacles).
• Barrier beaches - Long, narrow islands of sand or gravel parallel to the shore but detached, forming in areas with abundant sediment, gentle offshore slopes, powerful waves, and small tidal ranges. They likely originated post-ice age from rising sea levels depositing sand offshore, creating lagoons or marshes behind.

Role of vegetation in stabilising deposits

Vegetation enhances the stability of deposited sediment in coastal areas, making sites more resistant to erosion from waves and wind. Plants bind material and promote soil development through a process called succession.

How vegetation stabilises sediment

• Roots form networks that hold sediment particles together, preventing erosion.
• Upper plant parts reduce wind impact, stopping surface material from blowing away.
• Decaying plant matter adds organic content, aiding soil formation.

Many coastal plants are halophytic (salt-tolerant) and xerophytic (drought-tolerant), adapted to harsh conditions like sand dunes and saltmarshes.

Process of succession

Succession is the gradual change in an ecosystem over time, starting with bare sediment.

Stages of succession:

1. Pioneer species (small, hardy plants) colonise first, altering abiotic conditions (non-living factors like water availability) by dying and decomposing to create basic soil.
2. This improves the environment, allowing more complex plants to establish.
3. Over time, trees and diverse vegetation appear, leading to the climatic climax community (the stable, final plant group suited to local climate).

This process drives coastal accretion (land buildup) by stabilising deposits and encouraging further deposition.

Formation of sand dunes

Sand dunes develop behind beaches where wind transports sand inland from longshore drift deposits. Vegetation plays a crucial role in their growth and succession.

Stages of sand dune development

Sand accumulates around obstacles like driftwood or berms, colonised by pioneer species such as couch grass, forming embryo dunes (initial small mounds). Succession progresses inland:

• Fore dunes - Stabilised by grasses like marram grass, which trap more sand.
• Yellow dunes - Larger formations with some soil, supporting more plant variety.
• Mature dunes - Oldest and tallest (up to 13 metres), furthest inland, with trees like birch; dune slacks (low-lying wet areas) may form between ridges near the water table.

Dunes increase in age and height moving inland, as new embryo dunes continually form near the shore.

Formation of mudflats and saltmarshes

Mudflats and saltmarshes emerge in sheltered, low-energy coastal areas like estuaries or behind spits, through deposition of fine sediments.

Process of formation

Flocculation (the clumping and settling of sediment particles in water) allows silt and mud from rivers or tides to deposit, building mudflats. These are colonised by halophytic plants like eelgrass, which tolerate high salinity and tidal submergence. As vegetation traps more sediment, mudflats rise, forming saltmarshes that remain exposed longer between tides. Species diversity increases further from the sea as salt levels drop.

Features of mudflats and saltmarshes

Tidal currents or streams erode channels across their surfaces, which may flood at high tide or remain permanently wet. These landforms provide sheltered habitats, supporting gradual ecological development.